CC1: Climate trends

Introduction

Over the reporting period (2019–20 to 2022–23), the ACT’s climate has experienced wide variation between the extremely hot and dry conditions of 2019, to the wetter and cooler conditions from 2020 onwards. This variation has been driven by the change from the El Niño–Southern Oscillation to the La Niña cycle, as well as changing conditions in Indian Ocean Dipole and other drivers which influence climate across south-eastern Australia. Despite the return to cooler and wetter conditions, long term trends continue to show increased annual temperatures and the occurrence of extended periods of dry conditions in the ACT.

Observed surface temperature

There is a strong climate warming trend in the ACT. The annual mean maximum temperatures have been above the 1961 to 1990 average since 1997, with the exception of 2022 (Figure 1). Nine out of the ten hottest years in the ACT have occurred since 2006.

Extreme temperatures (above 35°C) are becoming more frequent in the ACT (Figure 2). The occurrence of days above 40°C have particularly increased in the last decade, with six of the nine years from 2013 to 2022 experiencing days above 40°C, compared to just six years from 1926 to 2012. The hottest year experienced in the ACT was in 2019 with 33 days over 35°C and six over 40°C.

Minimum temperatures have also warmed in the ACT. With the exception of 2012, every year since 1997 has been warmer than the 1961 to 1990 average (Figure 3). Six of the ten warmest mean minimum years have occurred since 2007.

The ACT’s warming climate is also shown in the increase in the number of days with minimum temperatures above 15°C (Figure 4) and the decrease in the number of days recording temperatures lower than 0°C (Figure 5).

Other ACT temperature trends include:^[2]Climate trends provided by the Bureau of Meteorology from the Canberra Airport monitoring station. Data period used is 1914 to 2022, excluding the years 1921-1925 when data was unavailable.

• Annual mean maximum temperatures have risen by 1.8°C from 1914 to 2022.
• The number of days above 35°C has increased by around six more days per year from 1914 to 2022.
• Annual mean minimum temperatures have risen by around 2.2°C from 1914 to 2022.
• There has been a decrease in the annual number of days below 10°C with around 21 fewer days since 1914 or two fewer days per decade (a decrease of around 70%).
• From 1961 to 1990, the annual average number of days below 10°C was 19. No year since 1995 has reached that number. Since 2008, there was only one year that has seen more than ten days below 10°C.
• The annual number of nights above 15°C per year has increased by around 27 from 1914 to 2022.
• Seven out of the top ten years for highest number of nights over 15°C have occurred since 2006.
• The annual number of nights below 0°C has decreased by around 28, an overall decrease of about 30% from 1914 to 2022, and
• Eight of the ten warmest springs on record for minimum temperatures have occurred since 2000.

Temperature trends over the reporting period (2019–20 to 2022–23) include:

• the ACT had its hottest day on record at 44°C on 4 January 2020
• 2019 was the warmest year on record for daytime temperatures
• 2019 had the record highest number of days over 35 C with 33 days
• 2019 had the record highest number of days over 40 C with six days
• 2019 had the record highest number of nights above 15°C with 53
• 2020 was the third warmest year on record for mean minimum temperatures
• 2020 was the warmest spring on record
• 2020 had the third fewest number of nights below freezing with 42, and 2022 had the fifth fewest (jointly with 2013) with 46 nights, and
• 2022 was the equal-third-warmest autumn for minimum temperatures.

These results demonstrate a clear warming climate trend in the ACT. The impacts of this warming are discussed in Indicator CC2: Impacts of climate change.

Observed rainfall

There is considerable year-to-year and decadal variability in annual rainfall averaged over the ACT region, with no long-term trend (Figure 6). Over the last two decades, the Millennium Drought drove drier-than-average conditions across the ACT from 2000 to 2009, before being broken by the two La Niña events of 2010–11 and 2011–12. The years 2013 to 2019 were again drier than average, except for 2016 which had a strong negative Indian Ocean Dipole in that year which typically results in wetter than average conditions over south-eastern Australia. More recently, three La Niña years from 2020 to 2022 resulted in above average rainfall across the ACT.

Significant rainfall events occurred on the 7–8 February 2020 with severe storms causing wide scale flooding in Canberra and its surrounds. The rainfall followed the 2020 Orroral Valley bushfire causing extreme levels of erosion in the recently burned areas. The erosion impacted greatly on terrestrial and aquatic ecosystems, and water quality in the ACT’s main drinking water catchment (see 4. Bushfires in the ACT). It also caused major damage to roads and infrastructure.

Projected climate trends

Regional climate modelling has identified the most significant effects of climate change on the ACT.^[3]Climate projections provided by the Bureau of Meteorology. These projections are dependent on the level of global greenhouse gas emissions. Projected impacts include:

• average temperatures will continue to increase with hotter temperatures (day and night), and longer and more frequent heatwaves
• an increase of up to 0.7°C for average, maximum and minimum temperatures by 2030 across all seasons
• days above 35°C will increase with up to an additional five hot days per year by 2030 and 20 more per year by 2070
• temperature increases will mainly occur in spring and summer, although the number of hot days will also extend into autumn
• an increase in severe fire weather and fire risk
• reduced rainfall, particularly for spring and winter rainfall, with more frequent and prolonged drought
• reductions in water for the natural environment and the community
• occurrences of extreme rainfall intensity are projected to increase with more frequent and severe storms over a longer summer season causing flash flooding, violent winds and thunderstorms
• greater run-off from storms will degrade water quality
• increased evapotranspiration will further decrease the amount of water available
• a decrease in cold nights below 2°C, and
• fewer frosts.

These projected changes will significantly increase climate pressures on the natural and urban environment.